1. Field of the Invention
The invention relates to an improved laser light heated probe and to a system and its components for controlling the temperature of a probe mounted at the distal end of an optical fiber to which laser light is transmitted to heat the probe. The invention has special application in angioplasty procedures wherein tissue is heated for the recanualization of occluded blood vessels by removal of intravascular plaque and thrombi therefrom. Other applications include the occlusion of unwanted blood vessels such as varicose veins and the heat treating or glazing of the internal lumen of blood vessels.
2. Description of the Related Art
An extensive review of related prior art is given in the related patent application Ser. No. 003,209. Such review is incorporated herein by reference but is summarized and supplemented here as necessary for background to the present invention. Since the immediate and primary application of the invention is directed to angioplasty procedures, the related art will be explained primarily in connection with that type of application recognizing however that the invention has even broader application than that described.
A medical laser system of a type useful in practicing the present invention is to be found in U.S. Pat. No. 4,633,872. Aspects of the present invention concern an improved temperature sensing and laser power control system for a laser heated probe and which may be used as a modification of the type of laser system described in the patent.
As referred to in the prior application, occlusive heart disease, involving blockage of important coronary arteries, is a major cause of death among persons in the adult population. Even where death does not occur as a result of such disease, occluded arteries in the extremities, liver, kidney, and brain nonetheless may lead to severe discomfort, loss of normal activity, and altering of the individual's quality of life.
The treatment of patients with occlusive arterial disease has generally been affected by two primary methods: pharmacological treatment for moderate arterial obstructions, and surgical treatments, including arterial bypass surgery and/or percutaneous transluminal angioplasty(PTA), in instances of severe stenosis. The benefits of PTA, as contrasted to bypass surgery, include less traumatic procedures, lower costs, respectable success in opening arterial occlusions, and relatively short patient recovery times. A primary disadvantage of PTA, however, is that the material causing the arterial blockage, e.g. arterial plaque or thrombi, is not removed but only pushed aside, with the possibility of future occlusions resulting from the continued accretion of plaque and/or thrombi to the displaced occluding deposits.
Laser irradiation has been proposed for the permanent removal of the aforementioned material deposits. The methods of laser irradiation which have been proposed include: (1) direct laser irradiation, in which intravascular plaque and thrombi are removed through abalation, and (2) indirect laser irradiation in which laser energy is converted to thermal energy through the interaction of a laser beam with an absorptive thermally conductive device, which when brought into close proximity or contact with plaque or thrombic complexes, dissolves, melts or evaporates the artery-blocking material.
The present invention insofar as it applies to laser angioplasty procedures is basically directed to the indirect laser irradiation technique and involves the insertion of an optical fiber into the arterial or venous channel, to function as a transmissive element for delivery of laser energy to a thermally conductive device for use at the treatment site. Cooling and temperature control of the probe are of critical concern and constitute the primary concerns of the invention.
Another concern of the invention is that of avoiding the introduction into the patient's body, particularly at the treatment site, of any type of electrical device which might create an electrical or magnetic perturbation capable of damaging or interferring with electrocardial functions. Another concern of the invention is ensuring that the thermally conductive device of the invention is suitably attached to the optical fiber such that it will not become disengaged during use. The invention is thus directed to dealing with each of these concerns in a manner not satisfactorily dealt with before by prior art indirect laser irradiation apparatus.
Federal Republic of Germany Pat. No. 2,826,383 published Dec. 20, 1979 is cited along with other references in U.S. Pat. Nos. 4,646,737 and 4,662,368. The earlier German patent as well as the two later U.S. patents disclose a heat generating element in the form of a metal probe mounted on an optical fiber through which light is transmitted to generate heat energy in the heat generating element. The German patent refers to cooling by flowing a gas within a casing to a location near the treatment site to extract heat from the heat generating element but does not disclose means for controlling the temperature of the heat generating element. The German patent mentions but does not describe or illustrate use of a thermocouple in a sheath tube surrounding the optical fiber. U.S. Pat. No. 4,646,737 also refers to use of a cooling fluid directed alongside the optical fiber as a means for cooling the heat generating element. U.S. Pat. No. 4,662,368 also refers to use of a cooling gas to cool the heat generating element.
U.S. Pat. Nos. 4,646,737 and 4,662,368 discuss a means for monitoring probe temperature by monitoring reflected infrared radiation emanating from a laser light heated probe during laser deactivation. Such a method does not lend itself to real time monitoring or control of probe temperature since the laser is turned off during the measurement or measurements made during the cooling cycle of the probe. In practice, probes as depicted in U.S. Pat. Nos. 4,646,737 and 4,662,368 have relied heavily on a dosimetry matrix for estimation of probe temperature in which prior in-vitro data was accumulated which relates probe temperature to input laser power level. Such a dosimetry matrix is highly variable and dependent upon the fluid dynamic environment surrounding the probe body. As a result, large deviations from predicted probe temperatures are possible. For reference see U.S. Pat. No. 4,662,368 and an article by George S. Abela et al entitled "Hot Tip: Another Method of Laser Vascular Recanalization" published in Surgery and Medicine 5:327-335, 1985.
The laser energized thermal probe described in prior application Ser. No. 003,209 teaches use of a coiled thermally conductive wire as a means of joining the probe body to the optical fiber and providing a substantially improved means for preventing the probe from becoming disengaged during use. This helically wound heat conductive wire also serves as a means for dissipating heat generated by the probe in use thus avoiding the need for cooling fluids and the like. In another aspect of the laser energized thermal probe of the prior application, there is provided a mass of a laser energy-absorptive, thermally conductive, high emissivity medium contained within the interior of the probe tip which functions to maximize the absorption of the laser energy by the probe.
What the present invention seeks to achieve is a still further improved laser heated probe by incorporating with the probe apparatus invention of the prior application, means for real time sensing and controlling the temperature of the heat generating element and controlling the laser power to maintain some predetermined temperature. Since the present invention is directed to providing a unique thermocouple controlled temperature sensing and temperature and power control system for an optical fiber mounted probe, mention is made of U.S. Pat. No. 4,476,512 in which a relatively low temperature measuring heat-sensitive element is positioned at the output end of an optical fiber as a means of sensing abnormal changes in the surface temperature of the fiber at its output end and thereby interrupting the laser power delivery should abnormal changes in optical fiber temperature be detected. The optical fiber and relative low temperature measuring heat-sensitive element lead are shown as separate elements. This patent also illustrates and discusses the prior art practice of using a measure of light entering the fiber for power control. The system of this patent is apparently directed to the direct laser irradiation technique since it makes no disclosure of using the light energy to heat a probe or the like. Furthermore, precise control of optical fiber surface temperature by varying input laser power is not addressed nor is the measurement of relatively high temperature of a probe heated by laser energy as with the present invention.
Electrically heated probes for vascular treatment are known. The article "The Heater Probe: A New Endoscopic Method For Stopping Massive Gastrointestinal Bleeding" by Robert L. Protell et. al. Gastroenterology 74, pages 257-262(1978), for example, describes an electrically heated, hollow, cylindrical metal probe which can be passed via an endoscope to apply pressure and heat simultaneously to a bleeding vessel. The electrically heated probe comprises a hollow aluminum cylinder with an inner heater coil wound on a ceramic core. A thermocouple element contained within the heater coil within the probe tip measures the actual temperature and feeds this information back to a servo-mechanism which maintains a selected temperature at the desired level. Such a probe inherently requires the presence of electrical current proximate the operative site, is large with respect to arterial and venous vessel diameters and is also characterized by having a relatively slow response time with respect to heating and cooling of the probe. Such a probe also has the disadvantage of being limited in the temperature to which the tip can be raised because of the wire size, power supply and like considerations.
What can be observed from the foregoing description is that the advent of the laser heated probe has provided a means for insuring permanent removal of blood vessel-occluding material. The laser heated probe disclosed in prior patent application Ser. No. 003,209 also illustrates that the blood vessel occluding material can be removed without requiring introduction of an external supplied coolant medium to the treatment site and without requiring the presence of an electrical current or electrical heater at the site.
Accordingly, it is an object of the present invention as with the invention of the prior application Ser. No. 003,209 to provide a laser heated probe operating at a relatively high temperature which is efficiently cooled through conductive means, does not require the introduction of an externally supplied coolant medium to the treatment site and provides a probe having a high degree of structural integrity.
A more specific object is to provide a thermocouple equipped laser heated probe forming part of an improved temperature sensing and laser power control system for applying the indirect laser irradiation technique particularly as it applies to laser angioplasty. The advantage of relatively simple calibration and of being able to readily monitor the sensing outputs for all temperatures likely to be encountered are thus other objects of the present invention as achieved through coupling the laser heated probe with the laser power control system via a temperature sensing thermocouple hereafter described.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.